Ultrasound imaging systems generate images of a body. One type of imaging is harmonic imaging. Ultrasound signals or pulses are transmitted at fundamental frequencies, and echo signals are received by a transducer. The echo signals are filtered to obtain signals associated with harmonic frequencies.
The signals associated with harmonic frequencies are generated through non-linear propagation and scattering within the body. For example, the signals are generated by non-linear scattering from contrast agents. Non-linear contrast agents are described for example by V. Uhlendorf, et al., in “Nonlinear Acoustical Response of Coated Microbubbles in Diagnostic Ultrasound” (1995 Ultrasonic Symposium, pp. 1559-1562). Such agents possess a fundamental resonant frequency. When they are insonified with high intensity ultrasonic energy at this fundamental frequency, they radiate ultrasonic energy at a harmonic of the fundamental frequency as well as at the fundamental frequency. Such contrast agents are often used to highlight regions containing blood loaded with the contrast agent. For example, in the case of a blood-filled chamber of the heart, the borders of the chamber can be distinguished more easily when contrast agents are used. Since the contrast agent generates harmonic ultrasound energy, echoes at the fundamental frequency from tissue or fluid containing no contrast agent may be eliminated by filtering at a receive beamformer.
Other than contrast agents, another source of harmonic energy is nonlinear propagation. As the ultrasonic burst propagates through the body, the burst distorts. The distortion corresponds to the shifting of energy to harmonic frequencies. As the burst propagates, more harmonic signals are generated. The harmonic signals are scattered from tissue and other structures in the body. Some of the harmonic signals impinge upon the transducer.
Another source of harmonic energy is system hardware nonlinearity. For example, high-voltage transmit circuitry may introduce harmonic energy in the transmit pulse before application to the transducer for transmission. Nonlinearity of the transducer may also add to harmonic content in the transmitted signal.
For contrast agent imaging, the signals output by the receive beamformer preferably include substantially only information from contrast agents (nonlinear scattering) and structure within the body (nonlinear propagation). The present invention is directed to improvements to reduce the harmonic information associated with structures in the body and system nonlinearity for harmonic imaging, whether imaging tissue or contrast agents.